1. Field of the Invention
This invention relates to an electric power steering apparatus and, more particularly, to an electric power steering apparatus enabled to detect a failure of a torque sensor incorporated in an electric power steering apparatus, especially, a failure of a shortcircuit of a pair of detection coils detecting steering torque.
2. Description of Related Art
A non-contact torque sensor is known, having a torque detection portion adapted to convert twist motion of a torsion bar, which is proportional to steering torque, into change in inductance of a pair of detection coils, a bridge circuit configured by a pair of detection coils and a pair of resistors to output a detection signal corresponding to change in the inductance, a main torque amplifier amplifying a main torque signal outputted by the bridge circuit, and a sub torque amplifier amplifying a sub torque signal outputted by the bridge circuit.
To enhance the fail safe function of the torque sensor, it has been desired to configure the electric power steering apparatus to automatically perform periodical operation tests on the torque sensor at every lapse of a predetermined period, for example, at initial diagnosis of a control unit of the electric power steering apparatus after an ignition key is turned to an ON-position, or during an operating state.
The torque sensor has a drawback in that torque cannot accurately be detected, especially, at occurrence of an abnormality, for example, at reduction in the amplitude of an AC voltage signal supplied to the bridge circuit constituted by a pair of detection coils and a pair of resistors in the torque sensor for some cause.
As a countermeasure against this drawback, it has been proposed a torque detection circuit which is suitable for use in the above torque sensor and which has a monitoring portion enabled to monitor the amplitude of an AC voltage signal to be supplied to the bridge circuit and also enabled to determine whether the torque detection circuit itself normally operates (see Japanese Patent Unexamined Publication JP-A-2002-48656)
FIG. 6 is a block diagram illustrating the configurations of the conventional torque detection circuit 100 and a peripheral circuit incorporated in a control circuit 200 that controls an electric power steering apparatus (not shown) associated with the torque detection circuit 100. An AC voltage Vosc is supplied to a bridge circuit constituted by a first arm, which has a detection coil L1 and a resistor R1, and a second arm having a detection coil L2 and a resistor R2. A main torque signal Vtm is obtained by amplifying a differential voltage Vdef and a voltage outputted from the connecting part between the detection coil L2 and the resistor R2 of the second arm through a main torque amplifying circuit. Note that the differential voltage Vdef represents the difference between a voltage outputted from the connecting part between the detection coil L1 and the resistor R1 of the first arm. Additionally, a sub torque signal Vts is obtained by amplifying the differential voltage Vdef through a sub torque amplifying circuit.
The torque detection circuit 100 is configured so that an abnormality thereof is detected by comparing the detected main torque signal Vtm with the detected sub torque signal Vts at an abnormality detection portion 105, and that when an abnormality is detected, the torque detection circuit 100 outputs an abnormality detection signal to change a mode of an electric power steering apparatus into a fail safe mode. Additionally, different reference voltages are supplied to a monitoring portion 103 from a reference voltage generation circuit (not shown). Thus, the monitoring portion 103 monitors whether an abnormality of the amplitude of the AC voltage signal Vosc occurs. Also, the monitoring portion 103 has a self-diagnostic mode to determine whether the monitoring portion 103 normally operates (see the JP-A-2002-48656).
In a case where a failure of an electric power steering apparatus occurs, usually, a main body and a control unit of the electric power steering apparatus are replaced at a service station. These collected units causing failures are analyzed at a manufacturing department. Results are used for study on measures to prevent recurrences of the failures and for future development.
Meanwhile, among various failures, a shortcircuit failure occurring between a main torque signal line and a sub torque signal line of a torque sensor is poor in reproducibility. Thus, even when the collected unit is operated under the same conditions as those under which a failure has occurred, the apparatus often fails to reproduce the failure. Fails being poor in reproducibility are not necessarily the shortcircuit failures occurring between the main torque signal line and the sub torque signal line. Therefore, the causes of the failure should be determined by performing various tests and checks. However, it takes significant cost and time to identify the failure as a shortcircuit failure occurring between the main torque signal line and the sub torque signal line. Accordingly, it has been earnestly desired to easily identify the failure as a shortcircuit failure occurring between torque signal lines.
Hitherto, in a case where a failure of the torque sensor is detected at initial diagnosis performed by the control unit just after the ignition key is turned to the ON-position, the mode of the apparatus has been changed to the fail safe mode in which a steering assisted by a motor is stopped. However, when the shortcircuit failure occurring between the main torque signal line and the sub torque signal line, neither a main torque signal nor a sub torque signal represents an abnormal value. Thus, in such a case, even when the steering assist is continued by using, for example, a main torque signal, no troubles occur.